Equation incl. number on the same line as theorem environment

Question

I have this:

\begin{lemma}
 \begin{equation} 
   L_b \circ L_a = L_{L_b(a)} \circ L_b.
 \end{equation}
\end{lemma}

and I would like the equation to be on the same line as the lemma, with a number label.
Adding \vspace{-\abovedisplayskip} doesn't work.
How can it be solved?

Answer 1

You can manually set this very specific requirement:

\documentclass{article}
\usepackage{amsmath,amsthm}
\newtheorem{lemma}{Lemma}
\begin{document}
Some regular text.
\begin{lemma}
This is a lemma.
\end{lemma}
Here is a break of some more text.
\begin{lemma}
\begin{equation}
  L_b \circ L_a = L_{L_b(a)} \circ L_b.
\end{equation}
\end{lemma}
Another break of regular text.

\medskip
\refstepcounter{lemma}%
\noindent\leavevmode\rlap{\textbf{Lemma~\thelemma.}}\hfill%
$L_b \circ L_a = L_{L_b(a)} \circ L_b.$\hfill%
\refstepcounter{equation}%
\llap{(\theequation)}\par%
\medskip

Some final text.
\end{document}

\medskip seemed sufficient in terms of the spacing. However, if need be, one can dig through the code and find the exact vertical spacing above/below the lemma to make it identical to the rest.

The overlapping (\rlap and \llap) is to make sure the equation is eventually centred when using \hfill.

Answer 2

One can also define a centredequ environment that centres the equation in the remaining free part of the current line. It works with the cleveref package:

\documentclass[11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}

\usepackage{amsmath,amsthm}
\newtheorem{lemma}{Lemma}

\usepackage{cleveref} 
\newcounter{centredequ}
\newenvironment{centredequ}{\refstepcounter{equation}\hfill\begin{math}}{\end{math}\hfill$(\theequation)$\par\noindent}
\crefname{centredequ}{eq.}{eqs. }
\Crefname{centredequ}{Eq.}{Eqs. }

\begin{document}

First a displayed equation: 
\begin{equation}\label{disp}
  a = b
\end{equation}
Some regular text with a centred equation: \begin{centredequ}\label{taut}
  a' = b'.
\end{centredequ}
\Cref{taut} is almost the same as \cref{disp}. 

\begin{lemma}
This is a lemma.
\end{lemma}
Here is a break of some more text.

\begin{lemma}\label{lem2}
\begin{centredequ}L_b \circ L_a = L_{L_b(a)} \circ L_b.\label{com}\end{centredequ}%
\end{lemma}
As we can see from \cref{taut} and \cref{lem2}, \cref{com} …

\end{document}